This invention relates to a magnetic recording medium having a magnetic recording coat formed on one side of a base and a back coat on the other side.
With the spread of video tape recorders (hereinafter called VTRs) in recent years, there has been growing demand for video tapes and other magnetic recording media more compact in size and capable of uninterrupted recording or playback for longer periods of time than before. This demand has developed a tendency toward smoother and thinner media. The resulting media, however, tend to run unsmoothly, wind up irregularly, or have inadequate strength. As a countermeasure and also as a means of satisfying the general requirement for better picture quality of video tapes, back coating the rear side of the tapes is now a wide-spread practice.
However, the conventionally back-coated magnetic tapes are not always satisfactory in performance, and their problems have hitherto been pointed out as below:
(a) Drop of the color signal-to-noise ratio due to back coating.
(b) Air inclusion with consequent cinching.
(c) Undesired scraping of the back coat.
(d) Damaging of long-playing tapes on loading upon or unloading from the VTR.
With the view to remedying these drawbacks, various finely divided inorganic pigments or binders have heretofore been introduced as possible components of the back coat. Nevertheless, none of the combinations of these inorganic pigment powders and binders have successfully corrected the drawbacks. Some combinations even failed to achieve an improvement in the running properties of the tapes, the end the back coating is originally intended to attain.
Generally, the binder suited for use in forming a back coat is one which thoroughly disperses inorganic pigment particles. Inadequate dispersion would make the resulting back coat surface uneven. when the tape is wound on a reel, this unevenness will be transferred from the back coat of one layer to the magnetic coat on the next layer and so forth, reducing the S/N ratio accordingly. Also, it lowers the pigment-reinforcing effect of the binder, adversely affecting the repetitive-running durability of the tape, and causing unwanted back-coat scraping and white dusting. As will be clear to one of ordinary skill in the art, a large volume of white dust formed in this way can lead to malfunction of the VTR. Among the binders known to be capable of producing good dispersion of inorganic pigment particles are pyroxylin and vinyl chloride-vinyl acetate copolymer. Even such a binder, as a component of a back coat, cannot achieve completely thorough dispersion of the inorganic pigment powder and fails to give a back-coated magnetic recording medium which settles all the problems of the prior art.